The present invention relates generally to power system communications, and more particularly to apparatus capable of simultaneously transmitting and receiving digital data signals both at high rates and over long distances through power lines and power line transformers, including AC, DC, coaxial cables, and twisted pair lines.
"Power-line Carriers" are well known in the field of power system communications. The principal elements of such power-line carriers are transmitting and receiving terminals, which include one or more line traps, one or more coupling capacitors, and tuning and coupling equipment. Detailed information regarding the description and typical composition of conventional power-line carriers may be found in Fundamentals Handbook of Electrical and Computer Engineering Volume II: Communication Control Devices and Systems, John Wiley & Sons, 1983, pp 617-627, the contents of which are incorporated herein by reference. A significant problem associated with prior art power-line carriers is their requirement for one or more line traps, one or more capacitors, one or more coupling transformers or carrier frequency hybrid circuits and frequency connection cables.
All traditional couplers incorporate a ferrite or iron core transformer which causes signal distortion due to the non-linear phase characteristic of the transfer function between the transmit coupler and the receive coupler. The distortion is created by the presence of magnetic core material which exhibits hysteresis. For distribution power-line carriers, the distortion is particularly severe because the signal must propagate through at least three such non-linear devices, the distribution transformer and two power-line couplers, that use ferrite core transformers. The distortion caused by these non-linear devices leads to envelope delay distortion, which limits communication speeds.
A line with a characteristic impedance Zo is ideally matched by terminations equal to Zo at both ends. Since Zo is primarily resistive at the frequencies of interest, the input impedance of the couplers should also be primarily resistive and equal to Zo at the carrier frequencies. A general configuration to achieve this is shown in FIG. 4. FIG. 4 is a schematic diagram of a phase shift linear coupler of the present invention. The coupler uses a serially connected equivalent capacitor, Ceq, on the primary of a transformer. The design is based on two principles. First, the resonance between the coupling capacitor, Ceq, and the primary winding inductance, L1, provides a low resistive impedance at the desired transmit carrier frequency. Second, Ceq has a large enough impedance at 60 Hz to block the line frequency. Although this basic approach is not new, previous efforts at achieving satisfactory impedance matching encountered problems, as discussed below.
The major shortcoming of previous designs resulted from the use of ferrite or iron core transformers in the signal couplers. The inductance, L1, is altered to some unknown value due to the non-linearity of the core. This results in a mistuning of the desired carrier frequency. Also, the impedance of the primary winding at the desired carrier frequency is no longer purely resistive. This may lead to a mismatch with respect to the line characteristic impedance. In recognition of this fact, other designs (FIGS. 1, 2) attempt to merely couple a signal onto a power line with a low transceiver input impedance by using a large coupling capacitor (approx. 0.5 uF). This results in a significant coupling loss of up to 20 dB at the carrier frequency. FIG. 3 is a graphical illustration of the frequency response characteristics of a traditional coupler which uses a magnetic-core transformer.
In view of the above, it is an object of the present invention to provide a power line communications apparatus which utilizes a novel phase shift linear power, phone, twisted pair, and coaxial line coupler for both transmission and reception. It is a further object of the present invention to provide power-line communication apparatus utilizing novel air-core transformers which can be used for phone line, coaxial, LAN and power line communication through power line transformers. It is an additional object of the present invention to provide a power line communication apparatus in which the primary coil of the transformer resonates with an associated coupling capacitor network in order to achieve resistive matching to approximately the lowest known value of the line characteristic impedance and to maximize stable signal transmission onto the line. This resonance effectively creates a band pass filter at carrier frequency.